1. Field of the Invention
The invention relates to the transmission of data acquired by a measurement while drilling (MWD) tool during the drilling of a wellbore and to the generation of electrical power to operate an MWD tool. More particularly, the invention relates to an integral mud flow telemetry modulator and turbine-generator for simultaneously generating continuous wave pressure signals while generating power for the modulator and for an electronic sensor package of an MWD tool.
2. State of the Art
Modern well drilling techniques, particularly those concerned with the drilling of oil and gas wells, involve the use of several different measurement and telemetry systems to provide data regarding the formation and data regarding drilling mechanics during the drilling process. In MWD tools, data is acquired by sensors located in the drill string near the bit. This data is either stored in downhole memory or transmitted to the surface using mud flow telemetry devices. Mud flow telemetry devices transmit information to an uphole or surface detector in the form of acoustic pressure waves which are modulated through the drilling fluid (mud) that is normally circulated under pressure through the drill string during drilling operations. A typical modulator is provided with a fixed stator and a motor driven rotatable rotor each of which is formed with a plurality of spaced apart lobes. Gaps between adjacent lobes present a plurality of openings or ports for the mud flow stream. When the ports of the stator and rotor are in direct alignment, they provide the greatest passageway for the flow of drilling mud through the modulator. When the rotor rotates relative to the stator, alignment between the respective ports is shifted, interrupting the flow of mud to generate pressure pulses in the nature of acoustic signals. By selectively varying the rotation of the rotor to produce changes in the acoustic signals, modulation in the form of encoded pressure pulses is achieved. Various means are employed to regulate the rotation of the rotor.
Both the downhole sensors and the modulator of the MWD tool require electric power. Since it is not feasible to run an electric power supply cable from the surface through the drill string to the sensors or the modulator, electric power must be obtained downhole. The state of the art MWD devices obtain such power downhole either from a battery pack or a turbine-generator. While the sensor electronics in a typical MWD tool may only require 3 watts of power, the modulator typically requires at least 60 watts and may require up to 700 watts of power. With these power requirements, it has become common practice to provide a mud driven turbine-generator unit in the drill string downstream of the modulator with the sensor electronics located between the turbine and the modulator.
The drilling mud which is used to power the downhole turbine-generator and which is the medium through which the acoustic pressure waves are modulated, is pumped from the surface down through the drill string. The mud exits the drill bit where it acts as a lubricant and a coolant for drilling and is forced uphole through the annulus between the borehole wall and the drill string. As the mud flows downhole through the drill string it passes through the telemetry modulator and the turbine-generator. As mentioned above, the modulator is provided with a rotor mounted on a shaft and a fixed stator defining channels through which the mud flows. Rotation of the rotor relative to the stator acts like a valve to cause pressure modulation of the mud flow. The turbine-generator is provided with turbine blades (an impeller) which are coupled to a shaft which drives an alternator. Jamming problems are often encountered with turbine powered systems. In particular, if the modulator jams in a partially or fully closed position because of the passage of solid materials in the mud flow, the downstream turbine will slow and reduce the power available to the modulator. Under reduced power, it is difficult or impossible to rotate the rotor of the modulator. Thus, while turbines generally provide ample power, they can fail due to jamming of the modulator. While batteries are not subject to power reduction due to jamming of the modulator, they produce less power than turbine-generators and eventually fail. In either case, therefore, conservation of downhole power is a prime concern.
U.S. Pat. No. 4,914,637 to Goodsman discloses a pressure modulator controlled by a solenoid actuated latching means which has relatively low power requirements. A stator with vanes is located upstream of a rotor having channels. As mud flows and passes over the vanes, the vanes impart a swirl to the mud which accordingly applies a torque to the rotor as the mud passes through the channels in the rotor. The rotor is prevented from rotating by a solenoid actuated latching device having a number of pins and detents. When the solenoid is energized, a pin is freed from a detent and the rotor is free to rotate through an angle of 45 degrees whereupon it is arrested by another pin and detent. When the rotor is arrested, it occludes the flow of mud until the solenoid is activated once again. Occlusion of the mud flow causes a pressure pulse which is detectable at the surface. The power requirement of Goodsman's modulator (approximately 10 watts) is low enough to be met by a downhole battery pack. However, since Goodsman's modulator is not motor driven, but rather mud flow driven, it depends on the hydraulic conditions of the drilling fluid which may vary considerably. Thus, the torque acting on the rotor will vary and interfere with signal generation. Moreover, in many instances, the torque is so great that undue strain is placed on the latching device subjecting it to severe wear and early failure.
A different approach to downhole energy conservation is disclosed in U.S. Pat. No. 5,182,731 to Hoelscher et al. The rotation of the rotor of the modulator is limited to two positions by fixed stops on the stator so that it can only rotate through an angle necessary to open or close the mud flow ports. A reversible D.C. motor coupled to the rotor is used to rotate the rotor to the open or closed position. A switching circuit coupled to the motor can also be used to brake the motor by shorting the current generated by the motor as it freely rotates. Power is conserved according to the theory that the on-duration of the motor is always relatively short.
In addition to considerations of power requirements, modulator design must also be concerned with the telemetry scheme which will be used to transmit downhole data to the surface. The mud flow may be modulated in several different ways, e.g. digital pulsing, amplitude modulation, frequency modulation, or phase shift modulation. Goodman's modulator achieves its energy efficiency in part by using amplitude modulation. Unfortunately, amplitude modulation is very sensitive to noise, and the mud pumps at the surface, as well as pipe movement, generate a substantial amount of noise. When the modulated mud flow is detected at the surface for reception of data transmitted from downhole, the noise of the mud pumps presents a significant obstacle to accurate demodulation of the telemetry signal. Hoelscher's modulator relies on digital pulsing which, while less sensitive to noise, provides a slow data transmission rate. Digital pulsing of the mud flow can achieve a data transmission rate of only about one bit per second. Comparatively, a modulated carrier wave signal can achieve a transmission rate of up to eight bits per second.